1. Field of the Invention
The present invention relates to a radial tire by which a lighter-weight structure is achieved without a deterioration in any of the performances of the tire, and in particular, to a radial tire which is suitable for use as a radial tire for a passenger vehicle.
2. Description of the Related Art
In recent years, the demand to reduce the weight of tires in order to improve the fuel economy of automobiles has become stronger. Many techniques have been disclosed as effective means for reducing the weight of a tire in which attention is focused on the steel cords for belt reinforcement using the metal wires as cords for the belts without being twisted. For example, Japanese Patent Application Laid-Open (JP-A) Nos. 8-218283 and 11-91311 disclose techniques of forming metal wires with spiral shapes or wave shapes. JP-A Nos. 4-19201 and 9-323503 propose techniques relating to use of single wire cords and the arrangement thereof. However, with all of these techniques, there is the concern that, under rigorous use, fracture of the metal wires or separation at the belt end portions may occur. Further, JP-A No. 11-291710 discloses a belt ply including strands in which metal wires which are not twisted are aligned. However, this technique is substantially the same as conventional twisted cord structures, and the effect of lightening the weight of the tire cannot be so much expected therefrom.
The applicant of the present application has previously proposed: a technique of suppressing the growth and propagation of cracks generated at the belt end portions by discretely disposing a plurality of monofilament bundles (JP-A No. 4-95505), a technique of improving belt durability by spacing apart bundles of a plurality of monofilaments, and in each of the bundles, creating an arrangement in which the monofilaments are slightly spaced apart locally (JP-A No. 4-95506); and an optimal embedding density of monofilament bundles each containing 5 to 6 filaments (JP-A No. 10-292275). However, when a structure approaches a state in which metal wires are aligned in a single row along the width direction of the belt layer, i.e., when the degree of flatness of the metal wire bundle becomes high, the belt rigidity decreases, and it is easy for the belt to be broken or for belt layer separation to occur, such that the durability of the tire deteriorates. Further, due to such a deterioration in belt rigidity, the rolling resistance at the time of high speed travelling increases. Thus, the above-described conventional techniques still have many problems which have not been solved.
In order to address the need to lighten the weight of the tire, untwisted metal wires are used in the belt for tire reinforcement, and at the same time, problems relating to the tire performances, such as the aforementioned problems of fracture of the metal wires, separation at the belt layer, a deterioration in rolling resistance, and the like are yet to be solved.